gistfile1.py

def f1(s):
    # Python 2: ok
    # Python 3: TypeError: ord() expected string of length 1, but int found (it returns int for binary string)
    # RPython : ok
    return ord(s[0])

def f2(s):
    # Python 2: ok
    # Python 3: ok
    # RPython : Blocked block -- operation cannot succeed: v1 = ord(v0)
    return ord(s[0:1])

def main(argv):
    f1(b'a')
    # f2(b'a')
    return 0


def target(*args):
    return main, None

if __name__ == "__main__":
    main([])

Reason: in RPython s[0] returns char, s[0:1] – string, they are different types.